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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>

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<script>
    function fn(str) {
        var arr = str.split('')
        for (var i = 0; i < arr.length; i++) {
            if (arr[i] >= 'a' && arr[i] <= 'z') {
                arr[i] = arr[i].toUpperCase()
            } else {
                arr[i] = arr[i].toLowerCase()
            }

        }
        return arr.join('')
    }
    var str = fn('HelloWorld')
    console.log(str);
</script>